Synthesis And Catalytic Performance Of Hierarchical Structure Microspheres Zeolites

Zeolites,possessing crystalline aluminosilicates frameworks and ordered networks of micropores?typically 0.3-1.0 nm?,have been found extensive industrial use as catalysts,adsorbents,and ion exchangers,owing to their large surface areas,uniform micropores,high adsorption capacities.Zeolite micropores are beneficial for the unique shape selectivity in catalysis;however,the sole presence of the microporous network also imposes significant diffusion limitation.The diffusion of the bulky molecules into and from the active sites confined in the zeolite crystals is impeded.Even when the size of micropores is larger than the molecules of the reactants and products,slow mass transport of these molecules through the pore system may be apparent.Diffusion limitation due to restricted access and low diffusion coefficients will lower the effectiveness of the zeolite and reduce the reaction rate.Furthermore,it may also cause pore blocking due to the large molecules or coke formation that contributes to deactivation of the catalysts.To overcome this problem,the synthesis of porous zeolites with hierarchical porous structures is a best way.Hierarchical zeolites,presenting additional mesoporosity into the microporous structures and larger external surface areas,have shown to be very attractive as heterogeneous catalysts for reactions involving bulky molecules.Hierarchically structured zeolites integrate at least two levels of porosity and present the advantages associated with each level of porosity,from accessibility to mass transport.They are categorized into two types:intracrystalline mesopores in the microporous zeolite crystals and intercrystalline mesopores in between the intergrown nano-sized zeolite crystals.In the present work,we show a method for preparing hierarchical CaA and ZSM-5 zeolites microspheres based on the blocking effect using presilanized mesoporous silica?MS?as a silica source with high surface area and abundant surface hydroxyl.The detailed results are summarized as follows:1.The organic species grafted on the surface of MS through Si-C covalentbond.In the course of crystallization process,the organic species grafted on the surface of MS integrated into the zeolite framework through Si-C covalent bond.Due to the the organic species and mesopores structure of the mesoporous silica,which blocks the growth of zeolite crystal,the intracrystalline defects sites inside the crystal or nanosized zeolite crystal are formed.2.A hierarchical zeolite CaA with microporous,mesoporous and macroporous structure was hydrothermally synthesized by a“Bond-Blocking”method using organo-functionalized mesoporous silica?MS?as a silica source.?1?The characterization by XRD,SEM/TEM and N₂ adsorption/desorption techniques showed that the prepared material had well-crystalline zeolite Linde Type A?LTA?topological structure,1-2?m microspherical particle morphologies,and hierarchically intracrystalline micro-meso-macropores structure.In which,the meso?macro?porosities can be adjusted by the variation of the silanization degree of mesoporous silica surface.The creation of more meso?macro?porosity with the preservation of the microporosity,which holds the active acid sites,results in a good catalytic performance for methanol dehydration to dimethyl ether.?2?The acidity and pore structure of zeolite catalyst are two main influence factors for the catalytic activity and selectivity in MTD reaction.With respect to acidity,DME formation is mainly related to sites with weak and medium acidity,and strong acid sites are recognized as a main cause for undesirable byproducts in DME synthesis.It is known,from NH₃-TPD results,that no strong acid sites over both microporous and hierarchical CaA catalysts,and thus they are just preferable in the MTD reaction.However,the MP-CaA catalyst with more acid sites displays lower catalytic activity and selectivity.This indicates that the acidity of CaA zeolite is not the main reason that influences catalytic activity and selectivity,whereas the catalytic behavior of CaA zeolitic catalysts for MTD strongly depends on the hierarchical pore structure of the catalysts.For the HP-CaA-3 sample,the conversion of methanol and DME selectivity remain at nearly 60%and 100%,respectively.However,the conversion of methanol and DME selectivity decreases only from 37.2%to7.8%and from 67%to 58%for the MP-CaA samples.The reason is due to the decrease in the diffusion path,thus improving diffusion in micropore channels,favoring the access to the active surface and decreasing the tendency of undesirable hydrocarbon formation by the introduction of meso?macro?porosity into the interior of zeolite crystals.These results have indicated the interconnectivity of these micro-,meso and macropores in the hierarchical CaA zeolites.3.The same as synthesis principle of hierarchical zeolite CaA,hierarchical ZSM-5 zeolite microspheres were prepared by silanized MS as silica source.?1?The ZSM-5 nano-aggregates microspheres with 2-3?m are composed of20-30nm size-tunable nanocrystals,and the size and different morphologies of nanocrystals changed significantly from silanized MS accompanied with the synthesis conditions,in which hydrophobic moieties in organosilane inhibited partly the growth of zeolite crystals.Spontaneously stacking of nanocrystals in the zeolitic crystallization based on degrees of silanization on MS surface results that the ZSM-5 samples have high external surface area and mesopore volume from inter-nanocrystals.?2?The acidity of the external surface was also detected through the DTBPy-adsorbed IR spectra because the bigger DTBPy?0.79nm?cannot enter the channels of ZSM-5.2%of Bronsted acid sites are accessible in microporous HNZ-5-0,while 67%of Bronsted acid sites of HNZ-5-2 are accessible and efficient,reflecting good enhancement of hierarchical structure of ZSM-5zeolite on accessibility of acid sites.?3?The good accessibility of acid sites for ZSM-5 with hierarchical structure can be a remarkable benefit for catalytic reactions,when larger reactant molecules are involved,in which diffusion constraints and/or adsorption of reactant molecules onto the acid sites are the main concerns.Here,the liquid phase benzylation of 1,3,5-trimethylbenzene with benzylchloride was employed as a probe reaction to investigate the structural and acidic properties of the hierarchical ZSM-5 samples,in which 2-benzyl-1,3,5-trimethylbenzene is one and only product because of chemically steric effect.In the reaction,for example,HNZ-5-0 sample converted only about 5.0%of benzylchloride,while HNZ-5-1,HNZ-5-2 and HNZ-5-3 converted 33.0,48.0 and 86%of benzylchloride under the reaction time reach 60 min.Hierarchical structure display enhanced catalytic activity,which indicates that these reactions are catalyzed on the surfaces of the mesopore surface that possess a large number of acid sites.